As a result of ever-increasing environmental regulations and cost pressure, there is at present a trend toward the preparation of PSAs containing small amounts, if any, of solvent. This objective can most simply be realized through the hotmelt technology. A further advantage is the shortening in production time. In hotmelt units, adhesives can be laminated to backings or release paper significantly more quickly, thus saving time and money.
Hotmelt technology, however, is imposing ever-higher requirements on the adhesives. For high-grade industrial applications, polyacrylates in particular are preferred on account of their transparency and weathering stability.
To produce acrylic hotmelts, conventionally, acrylic monomers are polymerized in solution and then the solvent is removed in a concentration process in an extruder. Beside the advantages in transparency and weathering stability, however, acrylic PSAs also have to meet stringent requirements in the area of shear strength. This is achieved by means of polyacrylates of high molecular weight, high polarity, and subsequent efficient crosslinking.
Crosslinking, however, is a limiting factor for these acrylic hotmelts. UV crosslinking is not very suitable, since the depth of penetration of the UV radiation is limited to a maximum of 70 xcexcm. This value reduces further if resins are included as bond strength promoter additives. An alternative technology available is that of electron beam crosslinking, although for acrylic compositions its crosslinking mechanism is extremely unfavorable, given that these PSAs are generally saturated, with no double bonds present. Consequently, this crosslinking is markedly inferior to the conventional thermal crosslinking for solventborne acrylic compositions, resulting in a lower level of thermal shear strength.
Another property for the coating of acrylic compositions from the melt is the phenomenon of the orientation of the polymer chains. For the properties of PSAs, this orientation of the macromolecules plays a significant part. As a result of the orientation, the corresponding polymers may acquire special properties, but generally at least acquire a planar anisotropy of the properties. Some examples, applying to polymers in general, of properties which can be influenced by the degree of orientation are strength and stiffness of the polymers and of the plastics produced from them, thermal conductivity, thermal stability, and anisotropic behavior with respect to permeability to gases and liquids (I.M. Ward, Structure and Properties of oriented polymers, 2nd ed. 1997, Kluwer, Dortrecht).
Interesting properties have likewise been found for oriented PSAs. For instance, the generation of a partial orientation in partly crystalline, rubber-based PSAs has already been described in U.S. Pat. No. 5,866,249. As a result of the anisotropic adhesive properties, innovative PSA applications are defined.
A disadvantage of the prior art PSAs is firstly that the acrylic hotmelts must be crosslinked in the process in order to obtain the orientation and secondly that the orientation slowly decreases over a prolonged period of time, as a result of structural relaxation.
It is an object of the present invention to provide PSA systems which have a predetermined profile of properties without additional crosslinking, said profile being retained over a prolonged period, without having to accept the disadvantages of the prior art. A further object is to specify a process for preparing such PSAs.
Surprisingly, and in a manner unforeseeable for the skilled worker, this object is achieved by the oriented, pressure sensitively adhesive systems as specified in the main claim. The subclaims relate to preferred developments of this PSA. Also claimed is a process for preparing PSAs of this kind, and their use.
The invention accordingly provides pressure sensitively adhesive systems at least comprising a pressure sensitive adhesive based on at least one block copolymer, the weight fractions of the block copolymers totaling at least 50% of the adhesive, one block copolymer being composed at least in part on the basis of (meth)acrylic acid derivatives (1), additionally at least one block copolymer comprising at least the unit P(A)-P(B)-P(A) comprising at least one polymer block P(B) and at least two polymer blocks P(A) (2), where
P(A) independently of one another represent homopolymer blocks or copolymer blocks of monomers A, the polymer blocks P(A) each having a softening temperature in the range from +20xc2x0 C. to +175xc2x0 C.,
P(B) represents a homopolymer block or copolymer block of monomers B, the polymer block P(B) having a softening temperature in the range from xe2x88x92130xc2x0 C. to +10xc2x0 C.,
the polymer blocks P(A) and P(B) are not homogeneously miscible with one another,
a feature of the pressure sensitively adhesive system of the invention being that it is oriented, possessing a preferential direction, with the refractive index nMD measured in the preferential direction being greater than the refractive index nCD measured in a direction perpendicular to the preferential direction.
In the text below the polymer blocks P(A) are sometimes referred to as xe2x80x9chard blocksxe2x80x9d and the polymer blocks P(B) as xe2x80x9celastomer blocksxe2x80x9d. Moreover, the block copolymers comprising the unit P(A)-P(B)-P(A) are referred to below as triblock copolymers, even when other blocks are present in the block copolymer.
By softening temperature in this context is meant, in the case of amorphous systems, the glass transition temperature and, in the case of semicrystalline polymers, the melting temperature. Glass transition temperatures are stated as results of quasistatic methods such as Differential Scanning Calorimetry (DSC), for example.
Preferably, one, more or all of the properties (1) and/or (2) indicated above for at least one block copolymer apply to two or more, or all, of the block copolymers present.
In one very preferred embodiment of the PSA systems, the difference xcex94n in the refractive indices, where xcex94n=nMDxe2x88x92nCD, is xe2x89xa71.10xe2x88x925.
Very preferably, the PSA systems have different stress-strain behavior in the lengthwise direction and in the crosswise direction.
Systems which have turned out to be particularly advantageous in the sense of the invention are those having a shrinkback of at least 5%. The shrinkback is measured in accordance with the test method described as method D in the experimental section. (The shrinkback of unbacked PSA strips is defined as the ratio of the length of a test strip, taken along the coating direction, shortly after coating and after one week.)
Particular preference is given to PSA systems in which the structure of at least one block copolymer, preferably of two or more or all the block copolymers, can be described by one or more of the following general formulae:
P(A)-P(B)-P(A)xe2x80x83xe2x80x83(I)
P(B)-P(A)-P(B)-P(A)-P(B)xe2x80x83xe2x80x83(II)
[P(B)-P(A)]nXxe2x80x83xe2x80x83(III)
[P(B)-P(A)]nX[P(A)]mxe2x80x83xe2x80x83(IV),
where n=3 to 12, m=3 to 12 and X is a polyfunctional branching unit, i.e., a chemical component via which different polymer arms are linked to one another,
the polymer blocks P(A) independently of one another represent homopolymer or copolymer blocks of the monomers A, the polymer blocks P(A) each having a softening temperature in the range from +20xc2x0 C. to +175xc2x0 C., and
the polymer blocks P(B) independently of one another represent homopolymer or copolymer blocks of the monomers B, the polymer blocks P(B) each having a softening temperature in the range from xe2x88x92130xc2x0 C. to +10xc2x0 C.
The polymer blocks P(A) as described in the main claim or in the advantageous embodiments may comprise polymer chains of one single type of monomer from group A or copolymers of monomers of different structures from group A. In particular, the monomers A used may vary in their chemical structure and/or in their side chain length. The polymer blocks therefore span the range between completely homogeneous polymers, via polymers of monomers with the same chemical parent structure but different chain length, and those with the same number of carbons but different isomerism, through to randomly polymerized blocks of monomers of different length with different isomerism, from group A. The same applies to the polymer blocks P(B) in respect of the monomers from group B.
The unit P(A)-P(B)-P(A) may be either symmetrical [corresponding to P1(A)-P(B)-P2(A) where P1(A)=P2(A)] or asymmetric [corresponding, for instance, to the formula P3(A)-P(B)-P4(A) where P3(A)xe2x89xa0P4(A), but where both P3(A) and P4(A) are each polymer blocks as defined for P(A)] in construction.
One advantageous embodiment is when at least one block copolymer, preferably two or more or all of the block copolymers, has a symmetrical construction such that polymer blocks P(A) which are identical in chain length and/or chemical structure and/or polymer blocks P(B) which are identical in chain length and/or chemical structure are present. P3(A) and P4(A) may differ in particular in their chemical composition and/or their chain length.
As monomers for the elastomer block P(B) it is advantageous to use acrylic monomers. For this purpose it is possible in principle to use any acrylic compounds which are familiar to the skilled worker and are suitable for the synthesis of polymers. Preference is given to choosing those monomers which result in the polymer block P(B), alone or in combination with one or more further monomers, having glass transition temperatures of less than +10xc2x0 C. Accordingly, the vinyl monomers may be chosen with preference.
For preparing the polymer blocks P(B) it is advantageous to use from 75 to 100% by weight of acrylic and/or methacrylic acid derivatives of the general structure
CH2=CH(R1)(COOR2)xe2x80x83xe2x80x83(V)
in which R1=H or CH3 and R2=H or linear, branched or cyclic, saturated or unsaturated alkyl radicals having from 1 to 30, particularly from 4 to 18, carbon atoms and/or up to 25% by weight of vinyl compounds (VI) which advantageously contain functional groups.
Acrylic monomers used with very great preference as compounds (V) as components of polymer blocks P(B) include acrylates and methacrylates with alkyl groups composed of from 4 to 18 carbon atoms. Specific examples of appropriate compounds, without wishing to be restricted by this enumeration, include n-butyl acrylate, n-pentyl acrylate, n-hexyl acrylate, n-heptyl acrylate, n-octyl acrylate, n-nonyl acrylate, lauryl acrylate, stearyl acrylate, stearyl methacrylate, the branched isomers thereof, such as 2-ethylhexyl acrylate and isooctyl acrylate, for example, and also cyclic monomers such as cyclohexyl or norbornyl acrylate and isobornyl acrylate, for example.
Furthermore, vinyl monomers from the following groups can optionally be used as monomers as defined for (VI) for polymer blocks P(B): vinyl esters, vinyl ethers, vinyl halides, vinylidene halides, and also vinyl compounds containing aromatic cycles and heterocycles in the xcex1 position. Here again, selected monomers which can be used in accordance with the invention may be mentioned by way of example: vinyl acetate, vinyl-formamide, vinylpyridine, ethyl vinyl ether, 2-ethylhexyl vinyl ether, butyl vinyl ether, vinyl chloride, vinylidene chloride, and acrylonitrile.
Further suitable, particularly preferred examples of monomers containing vinyl groups as defined for (VI) for the elastomer block P(B) include hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, N-methylolacrylamide, acrylic acid, methacrylic acid, allyl alcohol, maleic anhydride, itaconic anhydride, itaconic acid, benzoin acrylate, acrylated benzophenone, acrylamide, and glycidyl methacrylate, to name but a few.
In one preferred embodiment of the inventive systems one or more of the polymer blocks comprise one or more grafted-on side chains. There is no restriction as to whether such systems are obtained by a graft-from process (polymerizational attachment of a side chain starting from an existing polymer backbone) or a graft-to process (attachment of polymer chains to a polymer backbone by way of polymer-analogous reactions). In particular for the preparation of such block copolymers it is possible as the monomers B to use monomers functionalized in such a way as to allow a graft-from process for the graft attachment of side chains. Mention may be made here in particular of acrylic and methacrylic monomers which carry halogen functionalization or any other functional groups which permit, for example, an ATRP (Atom Transfer Radical Polymerization) process. In this context, mention may also be made of the possibility of introducing side chains into the polymer chains in a targeted manner by way of macromonomers. The macromonomers may in turn be constructed in accordance with the monomers B.
In one special embodiment of this invention, the polymer blocks P(B) have installed into them one or more functional groups which permit radiation-chemical crosslinking of the polymer blocks, in particular by means of UV irradiation or by irradiation with rapid electrons. With this objective, monomer units which can be used include in particular acrylic esters which contain an unsaturated alkyl radical having from 3 to 18 carbon atoms and containing at least one carbon-carbon double bond. For acrylates modified with double bonds, allyl acrylate and acrylated cinnamates are suitable with particular advantage. Besides acrylic monomers, vinyl compounds with double bonds which are not reactive during the (radical) polymerization of the polymer block P(B) can be used with very great advantage as monomers for the polymer block P(B). Particularly preferred examples of such comonomers are isoprene and/or butadiene, but also chloroprene.
Starting monomers for the polymer blocks P(A) are preferably selected such that the resulting polymer blocks P(A) are immiscible with the polymer blocks P(B), so that microphase separation occurs. Advantageous examples of compounds which are used as monomers A are vinyl aromatics, which may also be alkylated, methyl methacrylate, cyclohexyl methacrylate, isobornyl methacrylate and isobornyl acrylate. Particularly preferred examples are methyl methacrylate and styrene, this listing making no claim to completeness.
In addition, however, the polymer blocks P(A) may also be constructed as a copolymer composed of at least 75% of the above monomers A, leading to a high softening temperature, or of a mixture of these monomers, but containing up to 25% of monomers B, leading to a reduction in the softening temperature of the polymer block P(A). By way of example but not exclusively, mention may be made in this context of alkyl acrylates, which are defined in accordance with the structure (V) and the comments made in relation to it.
In a further advantageous embodiment of the inventive pressure sensitive adhesive, polymer blocks P(A) and/or P(B) are functionalized in such a way that thermally initiated crosslinking can be conducted. The following, among others, may advantageously be chosen as crosslinkers: epoxides, aziridines, isocyanates, polycarbodiimides and metal chelates, to name but a few.
One preferred characteristic of the block copolymers used for the PSA systems of the invention is that their molar mass Mn is between 25,000 and 600,000 g/mol, preferably between 30,000 and 400,000 g/mol, with particular preference between 50,000 and 300,000 g/mol. The fraction of the polymer blocks P(A) is advantageously between 5 and 49 percent by weight of the overall block copolymer, preferably between 7.5 and 35 percent by weight, with particular preference between 10 and 30 percent by weight. The polydispersity of the block copolymer is preferably less than 3, given by the ratio D of mass average Mw, to numerical average Mn of the molar mass distribution.
The ratios of the chain lengths of the block copolymers P(A) to those of the block copolymers P(B) are chosen very advantageously in such a way that the block copolymers P(A) are present as a disperse phase (xe2x80x9cdomainxe2x80x9d) in a continuous matrix of the polymer blocks P(B). This is preferably the case at a polymer block P(A) content of less than about 25% by weight. The domains may preferably be in the form of spheres or distorted spheres. The formation of hexagonally packed cylindrical domains of the polymer blocks P(A) is likewise possible in the context of the invention. In a further embodiment an asymmetric design of the triblock copolymers is the aim, with the block lengths of the terminal polymer blocks P(A) in linear systems being different. The spherical morphology is especially preferable if an increase in the internal strength of the PSA is required, and also for the purpose of improving the mechanical properties.
In one version which is particularly preferred inventively, the molecular weight Mn of the middle block P(B) is limited to 250,000 g/mol in order to facilitate coating of the PSA from the melt.
Furthermore, it may be advantageous to use blends of the abovementioned block copolymers with diblock copolymers P(A)-P(B), it being possible for the monomers used to prepare the corresponding polymer blocks P(A) and P(B) to be the same monomers as above. It may also be of advantage to add polymers Pxe2x80x2(A) and/or Pxe2x80x2(B) to the PSA composed of the block copolymers, especially of triblock copolymers (I), or the PSA composed of a block copolymer/diblock copolymer blend in order to improve its properties.
The invention accordingly further provides pressure sensitively adhesive systems with a preferential direction, wherein the pressure sensitive adhesive comprises a blend of one or more block copolymers with a diblock copolymer P(A)-P(B),
where the polymer blocks P(A) (of the individual diblock copolymers) independently of one another represent homopolymer or copolymer blocks of the monomers A, the polymer blocks P(A) each having a softening temperature in the range from +20xc2x0 C. to +175xc2x0 C.,
and where the polymer blocks P(B) (of the individual diblock copolymers) independently of one another represent homopolymer or copolymer blocks of the monomers B, the polymer blocks P(B) each having a softening temperature in the range from xe2x88x92130xc2x0 C. to +10xc2x0 C.,
and/or with polymers Pxe2x80x2(A) and/or Pxe2x80x2(B),
where the polymers Pxe2x80x2(A) represent homopolymers and/or copolymers of the monomers A, the polymers Pxe2x80x2(A) each having a softening temperature in the range from +20xc2x0 C. to +175xc2x0 C.,
where the polymers Pxe2x80x2(B) represent homopolymers and/or copolymers of the monomers B, the polymers Pxe2x80x2(B) each having a softening temperature in the range from xe2x88x92130xc2x0 C. to +10xc2x0 C.,
and where the polymers Pxe2x80x2(A) and Pxe2x80x2(B), respectively, are preferably miscible with the polymer blocks P(A) and P(B), respectively.
Where both polymers Pxe2x80x2(A) and polymers Pxe2x80x2(B) are admixed, they are advantageously chosen such that the polymers Pxe2x80x2(A) and Pxe2x80x2(B) are not homogeneously miscible with one another.
As monomers for the diblock copolymers P(A)-P(B) and for the polymers Pxe2x80x2(A) and Pxe2x80x2(B) respectively it is preferred to use the monomers of groups A and B already mentioned.
At least one diblock copolymer, preferably two or more or all of the diblock copolymers, has or have a molar mass Mn of preferably between 5,000 and 600,000 g/mol, more preferably between 15,000 and 400,000 g/mol, with particular preference between 30,000 and 300,000 g/mol. They advantageously possess a polydispersity D=Mw/Mn of not more than 3. It is favorable if the fraction of the polymer blocks P(A) in respect of the composition of the diblock copolymer is between 3 and 50% by weight, preferably between 5 and 35% by weight. The diblock copolymers as well may advantageously have one or more grafted-on side chains.
Typical concentrations in which diblock copolymers are used in the blend are up to 250 parts by weight per 100 parts by weight of higher block copolymers comprising the unit P(A)-P(B)-P(A). The polymer blocks P(A) and P(B) of the diblock copolymers may be constructed as homopolymer blocks and also as copolymer blocks. In accordance with the remarks made above, they are advantageously chosen so as to be compatible with the block copolymers P(A) and P(B), respectively, of the triblock copolymers. The chain length of the polymer blocks P(A) and P(B) respectively is preferentially chosen so that it does not exceed that of the polymer block with which it is preferentially miscible or associable, and is advantageously 10% lower, very advantageously 20% lower, than this. The B block may also be chosen such that its length does not exceed half the block length of the B block of the triblock copolymer.
Corresponding remarks apply to the polymers Pxe2x80x2(A) and Pxe2x80x2(B), respectively. These may be constructed as homopolymers and also as copolymers. In accordance with the remarks made above, they are advantageously chosen so as to be compatible with the block copolymers P(A) and P(B), respectively, of the triblock copolymers and/or of the diblock copolymers. The chain length of the polymers Pxe2x80x2(A) and Pxe2x80x2(B) respectively is preferentially chosen such that it does not exceed that of the polymer block with which they are preferentially miscible or associable, and is advantageously 10% lower, very advantageously 20% lower, than this.
In order to prepare the block copolymers it is possible in principle to use any polymerizations which proceed in accordance with controlled-growth or living mechanisms, including combinations of different controlled polymerization processes. Without making any claim to completeness, mention may be made here by way of example, in addition to anionic polymerization, of ATRP, nitroxide/TEMPO-controlled polymerization or, more preferably, the RAFT process; that is, in particular, processes which permit control over the block lengths, polymer architecture or else, but not necessarily, the tacticity of the polymer chain.
Radical polymerizations may be conducted in the presence of an organic solvent or in the presence of water or in mixtures of organic solvents and/or organic solvents with water, or without the solvent. It is preferred to minimize the amount of solvent used. The polymerization time for free-radical processes, depending on conversion and temperature, is typically between 4 and 72 hours.
In the case of solution polymerization, solvents used are preferably esters of saturated carboxylic acids (such as ethyl acetate), aliphatic hydrocarbons (such as n-hexane, n-heptane or cyclohexane), ketones (such as acetone or methyl ethyl ketone), special boiling point spirit, aromatic solvents such as toluene or xylene, or mixtures of the aforementioned solvents. For polymerization in aqueous media or in mixtures of organic and aqueous solvents, it is preferred to add emulsifiers and stabilizers for the polymerization. Where a radical polymerization method is chosen, it is advantageous to make use, as polymerization initiators, of customary radical-forming compounds such as peroxides, azo compounds, and peroxosulfates, for example. Initiator mixtures are also outstandingly suitable.
In a favorable procedure, radical stabilization is effected using nitroxides of type (VIIa) or (VIIb): 
where R3, R4, R5, R6, R7, R8, R9 and R10 independently of one another denote the following compounds or atoms:
i) halides, such as chlorine, bromine or iodine, for example,
ii) linear, branched, cyclic and heterocyclic hydrocarbons having from 1 to 20 carbon atoms, which may be saturated, unsaturated, or aromatic,
iii) esters xe2x80x94COOR11, alkoxides xe2x80x94OR12 and/or phosphonates xe2x80x94PO(OR13)2, where R11, R12, and R13 stand for radicals from group ii).
Compounds of formula (VIIa) or (VIIb) may also be attached to polymer chains of any kind (primarily in the sense that at least one of the abovementioned radicals constitutes a polymer chain of this kind) and may therefore be used to construct the block copolymers, as macroradicals or macroregulators.
More preferably, compounds of the following type are used as controlled regulators for the polymerization:
2,2,5,5-tetramethyl-1-pyrrolidinyloxyl (PROXYL), 3-carbamoyl-PROXYL, 2,2-dimethyl-4,5-cyclohexyl-PROXYL, 3-oxo-PROXYL, 3-hydroxylimine-PROXYL, 3-aminomethyl-PROXYL, 3-methoxy-PROXYL, 3-t-butyl-PROXYL, 3,4-di-t-butyl-PROXYL
2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO), 4-benzoyloxy-TEMPO, 4-methoxy-TEMPO, 4-chloro-TEMPO, 4-hydroxy-TEMPO, 4-oxo-TEMPO, 4-amino-TEMPO, 2,2,6,6,-tetraethyl-1-piperidinyloxyl, 2,2,6-trimethyl-6-ethyl-1-piperidinyloxyl
N-tert-butyl 1-phenyl-2-methylpropyl nitroxide
N-tert-butyl 1-(2-naphthyl)-2-methylpropyl nitroxide
N-tert-butyl 1-diethylphosphono-2,2-dimethylpropyl nitroxide
N-tert-butyl 1-dibenzylphosphono-2,2-dimethylpropyl nitroxide
N-(1-phenyl-2-methylpropyl) 1-diethylphosphono-1-methylethyl nitroxide
di-t-butyl nitroxide
diphenyl nitroxide
t-butyl t-amyl nitroxide.
A range of further polymerization methods in accordance with which the PSAs can be prepared in an alternative procedure may be chosen from the prior art: U.S. Pat. No. 4,581,429 A discloses a controlled-growth radical polymerization process which uses as its initiator a compound of the formula Rxe2x80x2Rxe2x80x3N-O-Y, in which Y denotes a free radical species which is able to polymerize unsaturated monomers. In general, however, the reactions have low conversion rates. A particular problem is the polymerization of acrylates, which takes place only with very low yields and molar masses. WO 98/13392 A1 describes open-chain alkoxyamine compounds which have a symmetrical substitution pattern. EP 735 052 A1 discloses a process for preparing thermoplastic elastomers having narrow molar mass distributions. WO 96/24620 A1 describes a polymerization process in which very specific radical compounds, such as phosphorus-containing nitroxides based on imidazolidine, are used. WO 98/44008 A1 discloses specific nitroxyls based on morpholines, piperazinones, and piperazinediones. DE 199 49 352 A1 describes heterocyclic alkoxyamines as regulators in controlled-growth radical polymerizations. Corresponding further developments of the alkoxyamines or of the corresponding free nitroxides improve the efficiency for the preparation of polyacrylates (Hawker, contribution to the National Meeting of the American Chemical Society, Spring 1997; Husemann, contribution to the IUPAC World Polymer Meeting 1998, Gold Coast).
As a further controlled polymerization method, atom transfer radical polymerization (ATRP) can be used advantageously to synthesize the block copolymers, in which case use is made preferably, as initiator, of monofunctional or difunctional secondary or tertiary halides and, for abstracting the halide(s), of complexes of Cu, Ni, Fe, Pd, Pt, Ru, Os, Rh, Co, Ir, Ag or Au (EP 0 824 111 A1; EP 826 698 A1; EP824 110 A1; EP 841 346 A1; EP 850 957 A1). The various possibilities of ATRP are further described in U.S. Pat. No. 5,945,491 A, U.S. Pat. No. 5,854,364 A and U.S. Pat. No. 5,789,487 A.
Additionally and advantageously, the block copolymer used in accordance with the invention may be prepared by an anionic polymerization. In this case the reaction medium used preferably comprises inert solvents, such as aliphatic and cycloaliphatic hydrocarbons, for example, or else aromatic hydrocarbons.
The living polymer is generally represented by the structure PL(A)-Me, in which Me is a metal from group I of the Periodic Table, such as lithium, sodium or potassium, and PL(A) is a growing polymer block of the monomers A. The molar mass of the polymer block to be prepared is determined by the ratio of initiator concentration to monomer concentration. In order to construct the block structure, first of all the monomers A are added for constructing a polymer block P(A), then by adding the monomers B a polymer block P(B) is attached, and thereafter, by renewed addition of monomers A, a further polymer block P(A) is polymerized on, thus forming a triblock copolymer P(A)-P(B)-P(A). Alternatively, P(A)-P(B)-M can be coupled by means of an appropriate difunctional compound. In this way, starblock copolymers (P(B)-P(A))n are also obtainable. Examples of suitable polymerization initiators include n-propyllithium, n-butyllithium, sec-butyllithium, 2-naphthyllithium, cyclohexyllithium or octyllithium, with this listing making no claim to completeness. Furthermore, initiators based on samarium complexes are known for polymerizing acrylates (Macromolecules, 1995, 28, 7886) and can be used here.
Moreover, it is also possible to use difunctional initiators, such as 1,1,4,4-tetraphenyl-1,4-dilithiobutane or 1,1,4,4-tetraphenyl-1,4-dilithioisobutane, for example. Coinitiators may likewise be used. Suitable coinitiators include lithium halides, alkali metal alkoxides or alkylaluminum compounds. In one very preferred version the ligands and coinitiators are chosen such that acrylic monomers, such as n-butyl acrylate and 2-ethylhexyl acrylate, can be polymerized directly and need not be generated in the polymer by a transesterification with the corresponding alcohol.
A very preferred preparation process conducted is a variant of the RAFT polymerization (reversible addition-fragmentation chain transfer polymerization). The polymerization process is described in detail, for example, in WO 98/01478 A1 and WO 99/31144 A1. Suitable for preparing triblock copolymers, with particular advantage, are trithiocarbonates of the general structure Rxe2x80x2xe2x80x3-S-C(S)-S-Rxe2x80x2xe2x80x3 (Macromolecules 2000, 33, 243-245), by means of which, in a first step, monomers for the end blocks P(A) are polymerized. Then, in a second step, the middle block P(B) is synthesized. Following the polymerization of the end blocks P(A), the reaction can be terminated and reinitiated. It is also possible to carry out polymerization sequentially without interrupting the reaction. In one very advantageous variant use is made, for example, of the trithiocarbonates (VIII) and (IX) or the thio compounds (X) and (XI) for the polymerization, in which xcfx86 can be a phenyl ring, which may be unfunctionalized or functionalized by alkyl or aryl substituents attached directly or by way of ester or ether bridges, or can be a cyano group, or can be a saturated or unsaturated aliphatic radical. The phenyl ring xcfx86 may optionally carry one or more polymer blocks, such as polybutadiene, polyisoprene, polychloroprene or poly(meth)acrylate, which may be constructed in accordance with the definition for P(A) or P(B), or polystyrene, to name but a few. Functionalizations can be, for example, halogens, hydroxyl groups, epoxide groups, groups containing nitrogen or sulfur, without this list making any claim to completeness. 
Use may also be made of thioesters of the general structure RIVxe2x80x94C(S)xe2x80x94Sxe2x80x94RV, particularly in order to prepare asymmetric systems. RIV and RV may be chosen independently of one another, and RIV can be a radical from one of the following groups i) to iv) and RV a radical from one of the following groups i) to iii):
i) C1 to C18 alkyl, C2 to C18 alkenyl, C2 to C18 alkynyl, in each case linear or branched; aryl, phenyl, benzyl, aliphatic and aromatic heterocycles.
ii) xe2x80x94NH2, xe2x80x94NH-RVI, xe2x80x94NRVIRVII, xe2x80x94NHxe2x80x94C(O)-RVI, xe2x80x94NRVI-C(O)-RVII, xe2x80x94NHxe2x80x94C(S)-RVI, xe2x80x94NRVI-C(S)-RVII, 
xe2x80x83in which RVI and RVII are radicals chosen independently of one another from group i).
iii) xe2x80x94S-RVIII, xe2x80x94Sxe2x80x94C(S)-RVIII, in which RVIII can be a radical from one of groups i) or ii).
iv) xe2x80x94O-RVIII, xe2x80x94Oxe2x80x94C(O)-RVIII, in which RVIII can be a radical from one of groups i) or ii).
In conjunction with the abovementioned polymerizations which proceed in accordance with controlled-growth, free-radical mechanisms it is preferred to use initiator systems further comprising other free-radical polymerization initiators, especially radical-forming azo or peroxo initiators which decompose thermally. In principle, however, any customary initiators which are known for acrylates are suitable for this purpose. The production of C-centered radicals is described in Houben-Weyl, Methoden der Organischen Chemie, Vol. E19a, p. 60ff. These methods are employed preferentially. Examples of radical sources are peroxides, hydroperoxides, and azo compounds. As some nonexclusive examples of typical radical initiators, mention may be made here of: potassium peroxodisulfate, dibenzoyl peroxide, cumene hydroperoxide, cyclohexanone peroxide, cyclohexylsulfonyl acetyl peroxide, di-tert-butyl peroxide, azodiisobutyronitrile, diisopropyl percarbonate, tert-butyl peroctoate, and benzpinacol. In one very preferred variant the radical initiator used is 1,1xe2x80x2-azobis(cyclohexylnitrile) (Vazo 88(copyright), DuPont(copyright)) or 2,2-azobis(2-methylbutanenitrile) (Vazo 67(copyright), DuPont(copyright)). It is also possible, furthermore, to use radical sources which release free radicals only under UV irradiation.
In the conventional RAFT process, polymerization is generally carried out only to low conversion rates (WO 98/01478 A1) in order to produce molecular weight distributions as narrow as possible. As a result of the low conversion rates, however, these polymers cannot be used as PSAs and in particular not as hotmelt PSAs, since the high residual monomer fraction adversely affects the technical adhesive properties; the residual monomers which contaminate the solvent recyclate in the concentration process, and the corresponding self-adhesive tapes would exhibit very high outgassing.
In accordance with the invention, the solvent is preferably stripped off under reduced pressure in a concentrating extruder, for which purpose it is possible, for example, to use single-screw or twin-screw extruders which preferentially distill off the solvent in different or identical vacuum stages and which possess a feed preheater.
For advantageous further development in accordance with the invention it is possible to admix tackifier resin to the block copolymer pressure sensitive adhesives. In principle, all resins which are soluble in the corresponding polyacrylate middle block P(B) can be used. Suitable tackifier resins include rosin and rosin derivatives (rosin esters, including rosin derivatives stabilized by disproportionation or hydrogenation, for example), polyterpene resins, terpene-phenolic resins, alkylphenol resins, aliphatic, aromatic, and aliphatic-aromatic hydrocarbon resins, to name but a few. The resins chosen are primarily those which are preferentially compatible with the elastomer block. The weight fraction of the resins in the block copolymer is typically up to 40% by weight, more preferably up to 30% by weight.
For one specific embodiment of the invention it is also possible to use resins which are compatible with the polymer block P(A).
In addition, it is possible optionally to add plasticizers, fillers (e.g. fibers, carbon black, zinc oxide, titanium dioxide, chalk, solid or hollow glass beads, microbeads of other materials, silica, silicates), nucleators, blowing agents, compounding agents and/or aging inhibitors, in the form of primary and secondary antioxidants, for example, or in the form of light stabilizers.
The internal strength (cohesion) of the PSA is preferably brought about by the physical crosslinking of the polymer blocks P(A). The physical crosslinking thus obtained is typically thermoreversible. For nonreversible crosslinking, the PSAs may additionally be subjected to chemical crosslinking. For this purpose the acrylic block copolymer pressure sensitive adhesives used for the reversible systems of the invention may optionally comprise compatible crosslinker substances. Examples of suitable crosslinkers include metal chelates, polyfunctional isocyanates, polyfunctional amines, and polyfunctional alcohols. Polyfunctional acrylates can also be used with advantage as crosslinkers for actinic irradiation.
For optional crosslinking with UV light, UV-absorbing photoinitiators are added to the polyacrylate block copolymers which are employed in the systems of the invention. Useful photoinitiators which can be used to very good effect include benzoin ethers, such as benzoin methyl ether and benzoin isopropyl ether, for example, substituted acetophenones, such as 2,2-diethoxyacetophenone (available as Irgacure 651(copyright) from Ciba Geigy(copyright)), 2,2-dimethoxy-2-phenyl-1-phenylethanone, dimethoxyhydroxyaceto-phenone, for example, substituted xcex1-ketols, such as 2-methoxy-2-hydroxypropiophenone, for example, aromatic sulfonyl chlorides, such as 2-naphthylsulfonyl chloride, for example, and photoactive oximes, such as 1-phenyl-1,2-propanedione 2-(O-ethoxycarbonyl)oxime.
The abovementioned photoinitiators and others which can be used, including those of the Norrish I or Norrish II type, may contain the following radicals: benzophenone, aceto-phenone, benzil, benzoin, hydroxyalkylphenone, phenyl cyclohexyl ketone, anthraquinone, trimethylbenzoylphosphine oxide, methylthiophenyl morpholine ketone, amino ketone, azobenzoin, thioxanthone, hexaarylbisimidazole, triazine, or fluorenone, it being possible for each of these radicals to be further substituted by one or more halogen atoms and/or one or more alkyloxy groups and/or one or more amino groups or hydroxyl groups. A representative overview is given by Fouassier: xe2x80x9cPhotoinitiation, Photopolymerization and Photocuring: Fundamentals and Applicationsxe2x80x9d, Hanser-Verlag, Munich 1995. For further details, Carroy et al. in xe2x80x9cChemistry and Technology of UV and EB Formulation for Coatings, Inks and Paintsxe2x80x9d, Oldring (Ed.), 1994, SITA, London can be consulted.
Also claimed is a process for preparing oriented pressure sensitive adhesive systems based on above-described block copolymers, said process being outstandingly suitable in particular for preparing the inventive, pressure sensitively adhesive systems. In this process the pressure sensitive adhesive is coated from the melt, and in particular is stretched in the process.
Coating takes place advantageously through a melt die or through an extrusion die onto a backing material, the orientation being produced by stretching of the pressure sensitive adhesive tape film.
The extrusion coating takes place preferably through an extrusion die. Extrusion dies can be subdivided into three categories: T dies, fishtail dies, and coathanger dies. The individual types differ in the design of their flow channel. For preparing oriented acrylic hotmelt block copolymer PSAs, especially the pressure sensitively adhesive systems described above, it is preferred to carry out coating onto a backing using a coathanger die, specifically such that a polymer layer is formed on the backing by a movement of the die relative to the backing. In the case of extrusion die coating, the gap width of the die at the exit is greater than the thickness of the applied PSA film. Orientation therefore comes about by stretching. The orientation of the PSA following coating is retained as a result of the self-organization of the PSA; that is, a physical network of hard block domains P(A) is formed. Analogously, an orientation of acrylic block copolymer PSAs may also be produced with the melt die coating process.
The nature of the orientation within the PSAs is dependent both on the die temperature and coating temperature and on the temperature of the backing material. The best orientation effects are obtained by depositing onto a cold surface. Therefore, the backing material should be cooled directly during coating, by means of a roll. The roll can be cooled by a film of liquid, from the outside, or from the inside, or by a cooling gas. Additionally, the coating temperature and die temperature should be lowered in order to obtain a high degree of orientation. The degree of orientation is freely adjustable by the die gap width. The thicker the PSA film expressed from the extrusion die, the greater the extent to which the adhesive can be stretched to a relatively thin PSA film on the backing material. This stretching operation may be adjusted freely not only by the freely adjustable die width but also by the web speed of the accepting backing material. The adhesive is oriented by the coating operation.
Furthermore, the design of the melt die or extrusion die may likewise control the extent of orientation. The coating speeds may also be mentioned in this context, since at higher speeds a higher shear rate is likewise introduced, and thus the orientation is intensified. The anisotropy may also be produced later on, by stretching an acrylic PSA on an extensible backing. Stretching may be carried out either monoaxially or biaxially.
As well as by the process, the orientation may also be controlled by the composition of the block copolymers. In general, the extent of orientation goes up as the overall molecular weight increases, as the hard block domain fraction P(A) increases, and as the polarity of the middle block P(B) increases, in each case assuming identical coating temperature, coating speed, and draw ratio.
The oriented acrylic hotmelts, as they are or in the form of blends, are coated onto a backing. Suitable backing materials include, in principle, BOPP, PET, nonwoven, PVC, metal foils, foam, or release papers (glassine, HDPE, LDPE). For the purpose of determining the shrinkback in the free film, coating onto a release paper or release film is recommended. Otherwise, any backing materials familiar to the skilled worker for producing transfer tapes or single-sided and double-sided PSA tapes can be used. The orientation of the adhesive can be measured using a polarimeter, by means of infrared dichroism, or by X-ray scattering. A further method, particularly preferred for the inventive PSAs, is the measurement of the shrinkback in the free film. These measurement methods are described in detail under the experimental test methods (method D).
In principle it is also possible to crosslink the PSAs used in accordance with the invention with electron beams. Typical irradiation apparatus which may be employed includes linear cathode systems, scanner systems, and segmented cathode systems, where electron beam accelerators are concerned. A detailed description of the state of the art, and the most important process parameters, can be found in Skelhorne, Electron Beam Processing, in Chemistry and Technology of UV and EB formulation for Coatings, Inks and Paints, Vol. 1, 1991, SITA, London. The typical acceleration voltages are in the range between 50 kV and 500 kV, preferably between 80 kV and 300 kV. The scatter doses employed range between 5 and 150 kGy, in particular between 20 and 100 kGy.
The invention further provides for the use of these oriented pressure sensitive adhesives as and for producing single-sided and double-sided pressure sensitive adhesive tapes, and also as transfer tapes.
Exemplary product structures may be advantageous for the PSA systems of the invention:
PSA systems with a single-layer product structure, the layer being composed of a PSA as described above;
PSA systems with a multilayer product structure, especially a two-layer or three-layer product structure, where at least one of the layers is composed of a PSA as described above and preferably has a thickness of at least 10 xcexcm, with particular preference at least 25 xcexcm, and where preferably one of the other layers is composed of an elastomer;
PSA systems comprising at least one backing or carrier layer.
With particular advantage the PSA systems of the invention may be used for producing punched products. The punched product may be produced from a backing material coated on one or both sides with an anisotropic pressure sensitive adhesive.
Punched products of this kind can be used as single-sidedly or double-sidedly adhering labels, for adhesive bonding in the home and in industry, especially in automotive construction, for all assembly purposes, in the medical segment (patches, wound coverings) and the like, to mention but a few examples of their application. The punched products can, generally speaking, be employed wherever adhesive labels and adhesive sheets are used. They are particularly appropriate in applications where there is emphasis on a clean, undamaged edge to the punched product.
Further possible PSA tape applications are described below. When cable harnesses are used in the engine compartment, the differences in temperature which occur are in some cases very great. Accordingly, acrylic block copolymer PSA tapes can be used with advantage for such applications. Unlike a standard commercial PSA, an oriented PSA will contract, when heated, by the shrinkback, described above and measured, and so will form a firm bond between the cables and the insulating nonwoven (shrink adhesive). The shrinkback effect can also be exploited in the context of adhesive bonds on convex surfaces. By applying a PSA tape to a convex surface, followed by heating, the PSA tape contracts and thus conforms to the convexity of the substrate. In this way, bonding is greatly facilitated and the number of air inclusions between substrate and tape is distinctly reduced. The pressure sensitive adhesive is able to apply the optimum activity. This effect may be assisted further by an oriented backing material. Following application, under heating, both the backing material and the oriented PSA shrink, so that the bonds on the convexity are completely free from stresses.
The PSAs of the invention likewise offer a wide scope for applications which advantageously utilize the advantages of the low extension in the longitudinal (machine) direction and also the possibility of shrinkback.
The property of preextension of the PSAs can also be utilized to outstanding effect. A further exemplary field of application for such highly oriented acrylic PSAs is that of strippable double-sided adhesive bonds. Unlike conventional strippable products, the oriented PSA is already preextended by several 100%, so that in order to remove the double-sided adhesive bond the PSA need only be extended by a few percent more in the direction of stretching. With particular preference, these products are produced with a layer thickness of several 100 xcexcm.